Detection of the authenticity of an electronic circuit or of a product containing such a circuit

ABSTRACT

A method of authenticating a slave device. The method includes initializing, by a host device, a charge retention circuit of the slave device, and receiving, by the host device, an indication of a discharge time of the charge retention circuit. The host device authenticates the slave device based on the received indication of the discharge time of the charge retention device.

BACKGROUND

Technical Field

The present disclosure generally relates to electronic circuits and,more specifically, to a method for verifying the authenticity or theorigin of a product (an accessory or a consumable) intended to cooperatewith a device.

Description of the Related Art

In many fields, it is desired to guarantee that a product, for example,an ink cartridge, a battery, an accessory, etc., to be used in a device,is an original or authentic product, that is, a product approved by thedevice manufacturer. To achieve this, an authentication key is generallystored in an electronic circuit associated with this product and isused, when the product is installed in the device or when it shouldcooperate therewith, to verify that the product is authentic. However,if the secret can be pierced and a manufacturer commercializes productswhich are not approved by the device manufacturer, however equipped withcircuits having the right key, the devices will consider these productsas being authentic.

BRIEF SUMMARY

In an embodiment, a method comprises: initializing, by a host device, acharge retention circuit of a slave device; receiving, by the hostdevice, an indication of a discharge time of the charge retentioncircuit; determining, by the host device, an authenticity of the slavedevice based on the received indication of the discharge time of thecharge retention device; and controlling, by the host device,interactions with the slave device based on the determined authenticityof the slave device. In an embodiment, the host device comparesinformation representative of time, generated by the charge retentioncircuit, with information indicative of an expected duration. In anembodiment, initializing the charge retention circuit comprisesswitching off a power supply of the slave device. In an embodiment, aduration of time in which the power supply of the slave device isswitched off is random. In an embodiment, the method comprises:transmitting, by the host device to the slave device, a voltagerepresentative of a charge level to be stored in the charge retentioncircuit; charging, by the slave device, of the charge retention circuit;switching off, by the host device, of a power supply of the slave devicefor a duration of time; measuring, by the slave device, a residualquantity of charges in the charge retention circuit at an expiration ofthe duration of time; transmitting, by the slave device to the hostdevice, information representative of the residual quantity; andcomparing, by the host device, said information with an expected valueof said information. In an embodiment, the method comprises: receiving,by the slave device, a value indicating to the slave device ano-response time-period; charging, by the slave device, of the chargeretention circuit; initializing of a time counter and switching off, bythe host device, of a power supply of the slave device; switching on thepower supply and periodically interrogating, by host device, of theslave device until the slave device responds to an interrogation;determining, by the host device, an elapsed duration based on when theslave device responded to the interrogation; and comparing thedetermined elapsed duration with an expected duration. In an embodiment,the method comprises transmitting, by the host device to the slavedevice, said value indicating the no-response time period. In anembodiment, the method comprises: transmitting, by a third device to thehost device and the slave device, said value indicating the no-responsetime period.

In an embodiment, a device comprises: one or more memories; aslave-device interface; and circuitry, which, in operation: initializescharge-retention circuits via the slave-device interface; authenticatesslave devices based on indications of discharge times ofcharge-retention circuits received via the slave-device interface; andcontrols interactions with slave devices based on slave-deviceauthentications. In an embodiment, wherein the circuitry, in operation,compares an indication of a discharge time with information indicativeof an expected duration. In an embodiment, the circuitry, in operation,initializes a charge-retention circuit by switching off a power supplyassociated with the charge-retention circuit. In an embodiment, aduration of time in which the power supply associated with thecharge-retention circuit is switched off is random. In an embodiment,the circuitry, in operation: transmits, via the slave-device interface,a voltage representative of a charge level to be stored in acharge-retention circuit; switches off a power supply associated withthe charge-retention circuit for a duration of time; and responds toreceipt of an indication of a residual quantity of charges associatedwith the charge-retention circuit at an expiration of the duration oftime by comparing said indication with an expected value. In anembodiment, the circuitry, in operation: initializes a time counter andswitches off a power supply associated with a charge-retention circuit;switches on the power supply and periodically interrogates a slavedevice until the slave device responds to an interrogation; determinesan elapsed duration based on when the slave device responds to theinterrogation; and compares the determined elapsed duration with anexpected duration. In an embodiment, the circuitry, in operation,transmits via the slave-device interface, a value indicating ano-response time period. In an embodiment, the circuitry, in operation,receives the value indicating the no-response period from a thirddevice.

In an embodiment, a device comprises: a charge-retention circuit; aninterface to couple to a host device; circuitry, which, in operation,responds to an initialization signal, by: initializing thecharge-retention circuit; generating an indication of a discharge timeof the charge-retention circuit; controlling communication with the hostdevice based on the determined indication. In an embodiment, thecontrolling communication with the host device comprises transmittingthe indication to the host device via the interface. In an embodiment,the initializing signal comprises a switching off of a power supplyassociated with the charge-retention circuit. In an embodiment, theinitialization signal comprises a voltage representative of a chargelevel to be stored in the charge-retention circuit, and the circuitry,in operation: charges the charge retention circuit; measures a residualquantity of charges in the charge-retention circuit at an expiration ofa duration of time; and transmits, to the host device via the interface,information representative of the residual quantity to be compared withan expected value. In an embodiment, the initialization signal comprisesa value indicative of a no-response time-period, and the circuitry, inoperation, responds to the initialization signal by: charging thecharge-retention circuit; ignoring interrogations received via theinterface until a charge-level of the charge-retention circuitcorresponds to an expiration of the no-response time-period.

In an embodiment, a system comprises: a host device having controlcircuitry; and a slave device having a charge-retention circuit, whereinthe control circuitry of the host device, in operation: controls one ormore signals to cause the slave device to initialize thecharge-retention circuit; receives an indication of a discharge time ofthe charge-retention circuit; determines an authenticity of the slavedevice based on the received indication of the discharge time of thecharge-retention device; and controls interactions with the slave devicebased on the determined authenticity of the slave device. In anembodiment, the host device is a printer and the slave device is aprinter cartridge. In an embodiment, the control circuitry, inoperation, compares the indication of the discharge time withinformation indicative of an expected duration. In an embodiment, thecontrol circuitry, in operation: transmits to the slave device, avoltage representative of a charge level to be stored in thecharge-retention circuit; switches off a power supply associated withthe charge-retention circuit for a duration of time; and responds toreceipt of an indication of a residual quantity of charges associatedwith the charge-retention circuit at an expiration of the duration oftime by comparing said indication with an expected value. In anembodiment, the control circuitry, in operation: initializes a timecounter and switches off a power supply associated with thecharge-retention circuit; switches on the power supply and periodicallyinterrogates the slave device until the slave device responds to aninterrogation; determines an elapsed duration based on when the slavedevice responds to the interrogation; and compares the determinedelapsed duration with an expected duration.

An embodiment provides a solution particularly adapted to circuits whichare not permanently powered.

An embodiment provides a method of verifying the authenticity of aproduct associated with a host device, wherein: a first electroniccircuit of the device initializes a charge retention circuit of a secondelectronic circuit of the product; and the first circuit interprets adischarge duration of the charge retention circuit to decide whether theproduct is authentic or not.

According to an embodiment, the first circuit compares informationrepresentative of time, delivered by the charge retention circuit, withan expected duration.

According to an embodiment, the first circuit starts assessing theauthenticity by switching off the power supply of the second circuit.

According to an embodiment, the duration for which the power supply ofthe second circuit is stopped is random.

According to an embodiment: the first circuit communicates to the secondcircuit a voltage representative of a charge level to be stored in thecharge retention circuit; the second circuit charges the chargeretention circuit; the first circuit switches off the power supply ofthe second circuit for a certain duration at the end of which the secondcircuit measures the residual quantity of charges in the chargeretention circuit; the second circuit communicates informationrepresentative of this residual quantity to the first circuit; and thefirst circuit compares this information with an expected value.

According to an embodiment: the second circuit receives a time valueduring which the second circuit should refrain from responding to thefirst circuit; the second circuit charges the charge retention circuit;the first circuit switches off the power supply of the second circuitand initializes a time counter; the first circuit switches back on thesecond circuit and periodically interrogates it until it responds; andas soon as the second circuit responds, the first circuit compares theelapsed duration with an expected duration, which is a function of thecharacteristics of the charge retention circuit of the second circuit.

According to an embodiment, said time value is communicated to thesecond circuit by the first circuit.

According to an embodiment, said time value is communicated to the firstcircuit and to the second circuit by a third circuit.

An embodiment also provides a system comprising at least one host deviceand at least one product associated with this host device, adapted tothe described method.

According to an embodiment, the device is a printer and the product isan ink cartridge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a very simplified representation in the form of blocks of anexample of a system of the type to which the embodiments which will bedescribed apply as an example;

FIG. 2 illustrates, in a block diagram, an embodiment of theauthentication method;

FIG. 3 is a simplified electric diagram of an example of an embodimentof a circuit capable of controllably retaining electric charges for atime measurement;

FIG. 4 is a graph illustrating the behavior of a charge retentioncircuit of an embodiment by application of an embodiment of anauthentication method;

FIG. 5 is a block diagram of an embodiment of an authentication method;and

FIG. 6 is a block diagram of another embodiment of an authenticationmethod.

DETAILED DESCRIPTION

In the ensuing description, numerous specific details are provided inorder to facilitate as much as possible understanding of the embodimentsprovided by way of example. The embodiments may be implemented with orwithout specific details, or else with other methods, components,materials, etc. In other cases, structures, materials, or operationsthat are well known are not shown or described in detail so that aspectsof the embodiments will not be obscured. Reference in the framework ofthe present description to “an embodiment” or “one embodiment” meansthat a given peculiarity, structure, or characteristic described inconnection with the embodiment is comprised in at least one embodiment.Hence, recurrence of phrases such as “in an embodiment” or “in oneembodiment” in various points of the present description does notnecessarily refer to one and the same embodiment. Moreover, thepeculiarities, structures, or characteristics may be combined in anyconvenient way in one or more embodiments.

The notations and references are here provided only for convenience ofthe reader and do not define the scope or the meaning of theembodiments.

The same elements have been designated with the same reference numeralsin the different drawings unless the context indicates otherwise.

For clarity, only those steps and elements which are useful to theunderstanding of the embodiments which will be described have been shownand will be detailed. In particular, the generation of the signalsexchanged between the circuits and their interpretation on the terminalside and on the transponder side have not been detailed, the describedembodiments being compatible with usual techniques of generation andinterpretation of the signals. In the following description, whenreference is made to terms approximately, about, and in the order of,this means to within 10%, for example to within 5%.

FIG. 1 very schematically shows, in the form of blocks, an example of asystem of the type to which the embodiments which will be describedapply. A host device 1 (HOST) is capable of receiving or of operatingwith one or a plurality of products 2, accessories (ACC), or consumables(CONS). As a specific example of application, the host device is aprinter and the product (consumable) is an ink cartridge. According toanother example, it is an electronic system (for example, a gameconsole, a cell phone, etc.) using accessories (for example, game pads,earphones, a hull, a case, etc.). More generally, it may be any type ofsystem based on the cooperation between a main device (host) and one ora plurality of accessories or consumables (products).

Manufacturers of consumables or accessories are generally looking for aprotection against the use of counterfeit or non-authentic accessoriesin order, among other things, to guarantee the quality and thereliability of the original products with respect to copies or “clones”for their users. It may, for example, be desired to avoid possiblecounterfeits. Reference will be made hereafter to copies to designatenon-authentic products, be they slavish imitations or more generallysimilar products capable of being used as authentic products.

The protection generally comprises a mechanism of authentication of anew product introduced into the host device, or even an authenticationeach time the device is powered on, or leaves the stand-by mode, or eachtime the product is used (for example, for each printing). In theexample of a printer, the printer and all cartridges are equipped withan electronic circuit adapted to such an authentication, for example, acryptographic processor or a program executed by a processor.

For example, as illustrated in FIG. 1, host device 1 comprises a circuit12 of microcontroller type (μC) capable of communicating over one or aplurality of address, control, and data buses 13 with one or a pluralityof memories 14 (MEM), one or a plurality of peripherals 15 (PER), forexample, the various circuits of device 1, and one or a plurality ofinput-output circuits 16 (E/S), among which a device capable ofcommunicating with products 2.

A product 2, be it a consumable or an accessory, comprises at least onecircuit 22, which may be protected, for example, of microcontrollertype, comprising the same type of components (not shown): a processor,volatile and non-volatile memories, an input-output interface towards abus of communication with device 1, etc.

In usual techniques, device 1 and products 2 share authentication keysstored in the memory, having the authentication procedures basedthereon.

In the embodiments described hereafter, the authentication is notperformed by exchange of authentication keys stored in the form ofdigital words, but based on electric parameters intrinsic to theelectronic circuit of the cartridge.

An embodiment uses a discharge speed of a charge storage element on theside of product 2. An embodiment controls an electric power supply ofthe electronic circuits 22 associated with products 2 to determine theauthenticity thereof.

FIG. 2 illustrates, in a block diagram, an embodiment of theauthentication method.

The electronic circuit of product 1, here symbolized by a block 10(HOST), controls products 2, more specifically electronic circuits 22 ofthe products forming slave circuits (SLAVE), not only via acommunication bus 13, but also by controlling their powering, that is,by controlling the power supply, for example, by providing a powersupply bus 17.

Each slave circuit 22 comprises a circuit 3 capable of retainingelectric charges in controllable fashion for a time measurement.Examples of circuits of this type are described in U.S. Pat. Nos.8,963,574; 8,872,177; 8,331,203; 8,320,176; 8,036,020; and 8,339,848.

FIG. 3 is a simplified electric diagram of an example of a circuit 3capable of controllably retaining electric charges for a timemeasurement.

Circuit 3 comprises a first capacitive element C1 having a firstelectrode 31 connected to a floating node F and having a secondelectrode 32 connected to a terminal 33 of application of a voltage, anda second capacitive element C2 having a first electrode 34 connected tonode F and having a second electrode 35 connected to a terminal 36 ofapplication of a voltage. Circuit 3 further comprises a third capacitiveelement C3 having a first electrode 37 connected to node F and having asecond electrode 38 connected to a terminal 39 of application of avoltage, and having its dielectric space designed, due to itspermittivity and/or to its thickness, to have a non-negligible leakagealong time. Capacitive element C1 has a charge retention capacitygreater than that of element C3, and capacitive element C2 has a chargeretention capacity greater than that of element C3, but smaller thanthat of element C1. A function of capacitive element C1 (storageelement) is to store electric charges. A function of capacitive elementC3 (flow element) is to discharge storage element C1 relatively slowlyas compared with a direct ground connection of its electrode 31. Afunction of capacitive element C2 is to allow a charge injection intocapacitive element C1, while avoiding the stress which would result, forflow element C3, from a direct charge of storage element C1 byapplication of a power supply voltage between node F and terminal 33.

In a charge retention phase initialization step, terminals 33 and 39 areat a reference voltage, for example, the ground, and a high power supplyvoltage (positive with respect to ground) Valim is applied to terminal36, which causes the charge of capacitive element C1. As a variation, tocharge element C1, terminal 39 may be grounded, and terminals 36 and 33may be set to voltages which are respectively positive and negative withrespect to ground. When the power supply voltage is no longer appliedbetween terminals 36 and 33, for example, when the circuit is no longerpowered, storage element C1 discharges in controlled fashion (relativelyslowly) through flow element C3. It should be noted that a controlleddischarge phase may also be provided when the circuit is still beingpowered. During the discharge phase, terminals 33, 36, and 39 may beleft floating, or even set to a same reference voltage, for example, theground. In a read phase, after a discharge phase, the residual charge ofstorage element C1 is measured (the device may be powered to take themeasurement). The residual charge of element C1 is considered asrepresentative of the time elapsed between the end of the initializationstep and the read step.

An embodiment exploits such electric charge retention circuits todetermine the authenticity of products 2.

In an embodiment, each time an authentication is needed (for example,each time a new cartridge is introduced into a printer, each time theprinter is powered on, each time the stand-by mode is left, for eachprinting, etc.), circuit 10 (FIG. 2) determines whether charge retentioncircuit 3 of circuit 22 behaves in expected fashion, that is, whetherits discharge time corresponds to an expected duration.

Circuit 10 thus contains, in its memory 14, information relative to thebehavior of circuits 3 of authentic products 2. For example, circuits 22are submitted to a test or characterization phase at the end of themanufacturing enabling to determine the discharge profile of theircircuits 3. According to another example, circuits 3 are formed in asufficiently reproducible way so that their time behavior can becharacterized.

FIG. 4 is a graph illustrating the behavior of a charge retentioncircuit 3 of a circuit 22 by application of an embodiment of anauthentication method.

It is assumed that circuit 10 causes the power supply of circuit 22 andthat, in a step of initialization of the measurement, it causes thecharge of circuit 3 up to a level representing a voltage V0 at node F ofcharge retention circuit 3.

Then, at a time T0, circuit 10 switches off the power supply of circuit22 for a time that it selects. This duration may be a fixed duration ora random duration.

At the end of this duration (time T1 in FIG. 4), circuit 10 switchescircuit 22 back on and asks for the charge value or residual voltagemeasurement. This measurement provides a level representing a voltageV1.

Circuit 22 then communicates measured value V1 to circuit 10, whichcompares this value with an expected value. This expected value is forexample obtained from a table stored in the memory of circuit 10. In thecase of a power supply cutoff of random duration, circuit 10 measuresthis duration to extract, from the characteristic of FIG. 4 stored inthe memory of circuit 10, the value of the voltage which should beprovided by circuit 22.

According to the duration (for example, T1 or Tn) for which the powersupply of circuit 22 is switched off, the voltage measurement (forexample, V1 or Vu) differs. It is thus possible for circuit 10 to knowafter how much time a voltage level (decreasing over time) should bereached or which voltage value is obtained at the end of a givenduration.

For the interpretation of the measurements, circuit 22 communicateseither voltage value V, or the corresponding time T, or estimates itselfa duration, as will be seen hereafter.

The response of charge retention circuit 3 is specific to the integratedcircuit chip forming circuit 3, included in circuit 22. Accordingly,this response differs from one chip to the other (actually from one chipcategory to another). A non-authentic cartridge having its circuit 22formed in a different technology will thus not provide the same responseand will not be authenticated.

According to the variability of the responses of circuits 3, forexample, due to manufacturing tolerances, approximate measurementthresholds taking such tolerances into account may be provided. As avariation, the characteristics of the responses of the differentcircuits may be communicated to the host devices with an identifier(code) of the products series.

FIG. 5 is a block diagram of an embodiment of the authentication method.

Master or host circuit 10 starts by communicating voltage V0 to slavecircuit 22 (block 51, V0→SLAVE). As a variation, voltage V0 is measuredby the slave circuit itself on reception of a control signal originatingfrom the master circuit. Then, circuit 10 switches off the power supplyof slave circuit 22 (block 52, SWITCH OFF 22). This interruption of thepower supply lasts for a determined or random time interval ΔT. At theend of time interval ΔT, circuit 10 switches circuit 22 back on (block23, SWITCH ON 22). As soon as it is switched back on, circuit 22estimates (block 54, SLAVE MEASURES T/V) the corresponding time T orvoltage V value, and then communicates this value to circuit 10 (block55, V or T→HOST). Circuit 10 compares (block 56, HOST COMPARES) thevalue transmitted by circuit 22 with an expected value, which is afunction of time interval ΔT.

The conversion between the voltage values and the time values is, forexample, performed using a conversion table stored on the master and/orslave side.

The authenticity is validated (block 57, DECISION) if this comparisonresults in an identity of the values (or an approximate identityaccording to the expected measurement tolerances and technologicaldispersions).

FIG. 6 is a block diagram of another embodiment of an authenticationmethod.

Master or host circuit 10 starts by communicating an indication of aduration T1 to slave circuit 22 (block 51, T1→SLAVE). This duration isstored by circuit 22 and represents a duration for which it refrainsfrom responding to any interrogation from circuit 10. Then, circuit 10switches off the power supply of circuit 22 and initializes a timecounter that it contains (block 52, SWITCH OFF 22, INIT TIMER). Circuit10 then periodically interrogates (63) circuit 22. For example, itswitches circuit 22 back on (block 631, SWITCH ON 22) and sends arequest (block 633, INTERROGATE 22). As long as circuit 22 does notrespond (output N of block 634, RESPONSE?), circuit 10 continuesmeasuring the time which elapses (loops back onto block 633). As soon ascircuit 22 responds (output Y of block 634), this means that the chargeretention circuit of circuit 22 has reached a charge level representingduration T1. Circuit 10 then reads the value of its time counter (block64, READ TIMER), then compares this value with the value that it hasinitially communicated (block 61) to circuit 22 to take an authenticitydecision (block 67, DECISION).

In the embodiment of FIG. 6, circuit 22 does not need to communicatedata to circuit 10, which is particularly adapted to simple circuits 22.It then just has to respond with an acknowledgement to an interrogationrequest to allow an authentication decision.

According to an alternative embodiment, duration T1 is not communicatedby the master circuit to the slave circuit, but this value orinformation representative of this value (encrypted value or cryptogram)is transmitted to the master circuit and to the slave circuit by athird-party circuit, possibly distant, for example generated by a dataserver with which circuits 10 and 22 are in remote communication.

Exchanges between the host device and the product(s) may be protected inusual fashion, for example, with a symmetrical or asymmetricalencryption based on keys contained in product 2 and in device 1.

Various embodiments have been described. Various alterations,modifications, and improvements will readily occur to those skilled inthe art. In particular, the selection of the value of the authenticationthresholds depends on the breakdown voltage of the transpondercomponents and may vary from one application to another. Further, thepractical implementation of the described embodiments is within theabilities of those skilled in the art based on the functionalindications given hereabove and by using electronic components usual perse.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present disclosure. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present disclosure is limited only as defined in thefollowing claims and the equivalents thereto.

Some embodiments may take the form of or include computer programproducts. For example, according to one embodiment there is provided acomputer readable medium including a computer program adapted to performone or more of the methods or functions described above. The medium maybe a physical storage medium such as for example a Read Only Memory(ROM) chip, or a disk such as a Digital Versatile Disk (DVD-ROM),Compact Disk (CD-ROM), a hard disk, a memory, a network, or a portablemedia article to be read by an appropriate drive or via an appropriateconnection, including as encoded in one or more barcodes or otherrelated codes stored on one or more such computer-readable mediums andbeing readable by an appropriate reader device.

Furthermore, in some embodiments, some of the systems and/or modulesand/or circuits and/or blocks may be implemented or provided in othermanners, such as at least partially in firmware and/or hardware,including, but not limited to, one or more application-specificintegrated circuits (ASICs), digital signal processors, discretecircuitry, logic gates, standard integrated circuits, state machines,look-up tables, controllers (e.g., by executing appropriateinstructions, and including microcontrollers and/or embeddedcontrollers), field-programmable gate arrays (FPGAs), complexprogrammable logic devices (CPLDs), etc., as well as devices that employRFID technology, and various combinations thereof.

The various embodiments described above can be combined to providefurther embodiments. Aspects of the embodiments can be modified, ifnecessary to employ concepts of the various patents, applications andpublications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

What is claimed is:
 1. A method, comprising: initializing, by a hostdevice, a charge retention circuit of a slave device, whereininitializing the charge retention circuit comprises switching off apower supply of the slave device; receiving, by the host device, anindication of a discharge time of the charge retention circuit;determining, by the host device, an authenticity of the slave devicebased on the received indication of the discharge time of the chargeretention circuit; and controlling, by the host device, communicationwith the slave device based on the determined authenticity of the slavedevice.
 2. The method of claim 1 wherein the host device comparesinformation representative of time, generated by the charge retentioncircuit, with information indicative of an expected duration.
 3. Themethod of claim 1 wherein a duration of time in which the power supplyof the slave device is switched off is random.
 4. The method of claim 1,comprising: transmitting, by the host device to the slave device, avoltage representative of a charge level to be stored in the chargeretention circuit; charging of the charge retention circuit; measuring aresidual quantity of charge in the charge retention circuit at anexpiration of a duration of time; and comparing information indicativeof the measured residual quantity of charge with an expected value ofsaid information.
 5. The method of claim 1, comprising: receiving, bythe slave device, a value indicating to the slave device a no-responsetime-period; charging, by the slave device, of the charge retentioncircuit; initializing of a time counter and switching off, by the hostdevice, of the power supply of the slave device; switching on the powersupply and periodically interrogating, by the host device, of the slavedevice until the slave device responds to an interrogation; determining,by the host device, an elapsed duration based on when the slave deviceresponded to the interrogation; and comparing the determined elapsedduration with an expected duration.
 6. The method of claim 5, comprisingtransmitting, by the host device to the slave device, said valueindicating the no-response time period.
 7. The method of claim 5,comprising transmitting, by a third device to the host device and theslave device, said value indicating the no-response time period.
 8. Adevice, comprising: one or more memories; a slave-device interface; andcircuitry, coupled to the slave-device interface, wherein the circuitry,in operation: initializes charge-retention circuits via the slave-deviceinterface, wherein initializing a charge retention circuit of a slavedevice comprises switching off a power supply of the slave deviceassociated with the charge retention circuit; authenticates slavedevices based on indications of discharge times of charge-retentioncircuits received via the slave-device interface; and controlscommunication with slave devices based on the authenticating of slavedevices.
 9. The device of claim 8 wherein the circuitry, in operation,compares an indication of a discharge time with information indicativeof an expected duration.
 10. The device of claim 8 wherein a duration oftime in which the power supply associated with the charge-retentioncircuit is switched off is random.
 11. The device of claim 8 wherein thecircuitry, in operation: transmits, via the slave-device interface, avoltage representative of a charge level to be stored in acharge-retention circuit; and switches off a power supply associatedwith the charge-retention circuit for a duration of time.
 12. The deviceof claim 8 wherein the circuitry, in operation: initializes a timecounter and switches off the power supply associated with acharge-retention circuit; switches on the power supply and periodicallyinterrogates the slave device until the slave device responds to aninterrogation; determines an elapsed duration based on when the slavedevice responds to the interrogation; and compares the determinedelapsed duration with an expected duration.
 13. The device of claim 12wherein the circuitry, in operation, transmits via the slave-deviceinterface, a value indicating a no-response time period.
 14. The deviceof claim 12 wherein the circuitry, in operation, receives the valueindicating the no-response period from a third device.
 15. A device,comprising: a charge-retention circuit; an interface to couple to a hostdevice; circuitry, which, in operation, responds to an initializationsignal, by: initializing the charge-retention circuit, wherein theinitializing signal switches off of a power supply associated with thecharge-retention circuit; generating an indication of a discharge timeof the charge-retention circuit; and controlling communication with thehost device based on the determined indication.
 16. The device of claim15 wherein the controlling communication with the host device comprisestransmitting the indication to the host device via the interface. 17.The device of claim 15 wherein the initialization signal comprises avoltage representative of a charge level to be stored in thecharge-retention circuit, and the circuitry, in operation: charges thecharge retention circuit; measures a residual quantity of charges in thecharge-retention circuit at an expiration of a duration of time; andtransmits, to the host device via the interface, informationrepresentative of the residual quantity.
 18. The device of claim 15wherein the initialization signal comprises a value indicative of ano-response time-period, and the circuitry, in operation, responds tothe initialization signal by: charging the charge-retention circuit;ignoring interrogations received via the interface until a charge-levelof the charge-retention circuit corresponds to an expiration of theno-response time-period.
 19. A system, comprising: a host device havingcontrol circuitry; and a slave device having a charge-retention circuit,wherein the control circuitry of the host device, in operation: controlsone or more signals to cause the slave device to initialize thecharge-retention circuit, wherein initializing the charge retentioncircuit comprises switching off a power supply of the slave device;receives an indication of a discharge time of the charge-retentioncircuit; determines an authenticity of the slave device based on thereceived indication of the discharge time of the charge-retentioncircuit; and controls communications with the slave device based on thedetermined authenticity of the slave device.
 20. The system of claim 19wherein the host device is a printer and the slave device is a printercartridge.
 21. The system of claim 19 wherein the control circuitry, inoperation, compares the indication of the discharge time withinformation indicative of an expected duration.